1. Field of the Invention
The present invention relates to an exposure apparatus, a light source apparatus and a method of manufacturing a device.
2. Description of the Related Art
In recent years, microfabrication techniques for projecting and transferring a finer pattern onto a substrate by an exposure apparatus available for a semiconductor manufacturing process are making remarkable progress. Under the circumstance, the wavelength of the exposure light is shortening, aiming at further improving the resolution of the exposure apparatus. As the wavelength of the exposure light shortens, the exposure light sources have shifted to KrF and ArF excimer lasers serving as kinds of gas lasers.
Because such an excimer laser can select only specific wavelengths by a narrow band module, it can achieve emission of specific pulsed light in a very narrow wavelength range by exploiting an oscillation phenomenon. Still better, the specific wavelengths can set a preset spectral width. Japanese Patent Laid-Open No. 2006-024855 discloses a technique of generating light with specific wavelengths packed into a very narrow band by a narrow band module, and correcting the wavefront aberration of the light emerging from the narrow band module, thereby changing the spectral width.
A conventional exposure apparatus uses light from an excimer laser for the following two purposes. One purpose is to project and transfer the pattern of a reticle onto a semiconductor wafer using pulsed light emitted by an excimer laser. Another purpose is to measure the position of a wafer stage via a reticle and a projection lens using light from an excimer laser.
Since the above-mentioned excimer lasers are gas lasers, periodical gas exchange is performed. This gas exchange often fluctuates the composition ratio of the gas inside the gas chamber, and, in turn, fluctuates the spectral distribution of the pulsed light. In addition, the characteristics of optical components present in the above-mentioned narrow band module also often change with time. This again accounts for a fluctuation in the spectral distribution.
Furthermore, although the above-mentioned excimer lasers have a function of maintaining the spectral width of the light constant, its spectral distribution fluctuates, as shown in FIG. 2, due to various factors associated with the internal environment of the excimer laser and, especially, with the oscillation frequency. Japanese Patent Laid-Open No. 2004-288874 points out this issue as well. This patent reference reveals that factors associated with the number of times of emission per second (frequency) result in acoustic waves, which adversely affect, for example, the spectral line width and the energy of the laser light.
As described above, the spectral distribution of an excimer laser used in an exposure apparatus often fluctuates due to various factors. Fluctuations attributed to these factors often adversely affect the pattern transfer performance of the exposure apparatus. When the spectral distribution of the excimer laser fluctuates, the contrast of a pattern to be projected and transferred naturally changes. When the contrast of that pattern changes, not only the contrast of the transferred pattern lowers but also the transferred pattern deforms because the degree of influence of that change differs among individual pattern elements to be transferred.
The technique described in Japanese Patent Laid-Open No. 2006-024855 can correct the spectral distribution which changes depending on the laser oscillation frequency to a target distribution by adjusting the position of a wavefront aberration adjusting mechanism included in the narrow band module. However, the exposure apparatus often uses different laser oscillation frequencies for exposure and for measurement of the position of the wafer stage. Laser oscillation for position measurement is performed at the maximum oscillation frequency of the laser because the completion of measurement in a short period of time leads to enhancement of the productivity of semiconductors. Laser oscillation for exposure is to expose a resist to light, so the exposure dose changes depending on, for example, the resist sensitivity. Hence, laser oscillation for exposure is sometimes performed at a relatively low frequency instead of the maximum frequency of the laser to suppress the exposure dose to be small.